This invention relates generally to methods and means for refurbishing worn turbine components, and more particularly to methods and means for refurbishing the shrouds of worn turbine blades.
After a period of operation, the blades employed in turbine engines and the like tend to become worn, especially in the vicinity of the trailing edge of the airfoil surface and at the location of the shrouds which serve as mountings for ends of the blades. The circularly arranged blades of a turbine stage extend radially and are supported at their outer ends by an air seal ring which engages the blade shrouds, while at their inner ends the blades are clamped in a suitable hub.
In the past, when the shrouds of the blades have become worn, they were usually refurbished by building up one set of edges of all the blades, usually a corresponding set of edges which all faced in the same direction when the blades were assembled in radial fashion, and then ground down an extent so as to produce a surface on each set of edges which was slightly higher than that of the original. Other edges which were worn only minimally were allowed to remain so. The reasoning was that the lesser worn edges could be used as a reference in providing an indication as to how much of the built-up edges should be removed.
Problems occurred when such procedures were employed, however. Ideally the shrouds were refurbished with respect to the stacking line of the blade, which is an imaginary line extending through a central portion of the blade longitudinally. When accurately restored with respect to this stacking line, the shrouds of the successive blades can nest with one another in the turbine rotor. However, improper build-up of material on one set of edges of the shrouds gave rise to dissymmetry of the shrouds with respect to these stacking lines, tending to forcibly shift the outer portions of the blades in a circumferential direction when the blades were installed. Since the inner, base portions of the blades were firmly held in the hub, there resulted undue stresses being developed in the airfoil structure; frequently this structure would crack transversely if the stresses which occurred were of sufficient magnitude. Such failures, due to imprecise refurbishing of the turbine blade shrouds, often had catastrophic results and accordingly could not be tolerated.
Where attempts were made to add welding material to all of the edges of the shroud, accurate refinishing of these built-up edges was difficult to achieve since there was no reference on the built-up blade by which to gauge the first grind. Frequently the airfoil structure of the blade was worn, and thus any attempt to use this airfoil as a reference in making the first grind on the built-up shroud would result in imprecise restoration, leading to further problems when the blades were re-installed in the turbine.